In the motion picture industry, a variety of techniques and systems are currently employed in the capture of scenes (light). Basically, three distinct classes of capture systems are used for the origination of images: traditional motion picture film systems, electronic systems and combinations of both systems (i.e., hybrid systems). When an electronic or hybrid capture system is chosen for the origination of images, it is common practice to apply some sort of processing to the images before they are displayed. In many cases, the image processing is done in order to mimic the “film look” so that electronically originated images can be perceived, when displayed, as if they were originally captured onto film. Several examples in the prior art can be found for electronic capture devices and methods which attempt to emulate the “film look”, e.g., see U.S. Pat. No. 4,935,816 (Method And Apparatus For Video linage Film Simulation) and U.S. Pat. No. 5,475,425 (Apparatus And Method For Creating Video Outputs That Emulate The Look Of Motion Picture Film).
Because traditional film systems have been used in the majority of cinematographic productions in the past decades, the “film look” has been chosen as the preferred look in many electronic applications. Generally, images captured by film systems differ somewhat from the “original scene” contrast and colorimetry. This discrepancy arises from the spectral differences between the human visual system—commonly represented by a set of color-matching functions—and the set of spectral sensitivity curves of the film used in conjunction with chemical image processing. FIG. 1 illustrates this point by co-plotting two such sets of curves balanced for a particular light source: curves 10, 12 and 14 represent a film response for red, green and blue, respectively, and curves 16, 18 and 20 represent the 1931 CIE (Commision Internationale De L'Eclairage) color matching functions for a set of primaries {overscore (x)}(λ),{overscore (y)}(λ),{overscore (z)}(λ) that correspond to the human visual system. Therefore, a capture system which “perceives” light in the same way as the human visual system offers the possibility of high colorimetric reproduction accuracy. In order to be able to quantify differences between the human visual system (HVS) and a traditional film system, it is helpful to briefly review a few notions of colorimetry and image science.
If a surface (object) reflects light according to a function Ref(λ), where λ represents wavelength values of visible light, this object is said to have a spectral reflectance given by Ref(λ). Consequently, in order to quantify light reflected from the object and captured by a specific capture system, the following additional elements must be accounted for:                I(λ): the spectral power distribution of the light source used to illuminate the object.        SRc(λ): the set of spectral response curves pertinent to the capture device. The index c refers to different “light channels” in the capture system. In the case of the human visual system or a film system, c assumes each of three values: red (r), green (g), or blue (b).Mathematically, these elements are related as follows:        
                                                        R              =                                                k                  r                                ·                                                      ∫                                          λ                      -                      visible                                                        ⁢                                                                                                              SR                          r                                                ⁡                                                  (                          λ                          )                                                                    ·                                              I                        ⁡                                                  (                          λ                          )                                                                    ·                                              Ref                        ⁡                                                  (                          λ                          )                                                                                      ⁢                                          ⅆ                      λ                                                                                                                                              G              =                                                k                  g                                ·                                                      ∫                                          λ                      -                      visible                                                        ⁢                                                                                                              SR                          g                                                ⁡                                                  (                          λ                          )                                                                    ·                                              I                        ⁡                                                  (                          λ                          )                                                                    ·                                              Ref                        ⁡                                                  (                          λ                          )                                                                                      ⁢                                          ⅆ                      λ                                                                                                                                              B              =                                                k                  b                                ·                                                      ∫                                          λ                      -                      visible                                                        ⁢                                                                                                              SR                          b                                                ⁡                                                  (                          λ                          )                                                                    ·                                              I                        ⁡                                                  (                          λ                          )                                                                    ·                                              Ref                        ⁡                                                  (                          λ                          )                                                                                      ⁢                                          ⅆ                      λ                                                                                                                                                    Eq            .                                                  ⁢            1                    ⁢          a                ,                  1          ⁢          b                ,                  1          ⁢          c                    
In Equations 1a, 1b and 1c, the constants kr,kg,kb are used for normalization purposes and the integrals are performed over a range of wavelength values which correspond to visible light. For the purpose of the present invention, the wavelength values (λ) used to represent visible light are given by the range: 380 nm≦λ≦720 nm. If SRr,SRg,SRb represent, respectively, a set of color-matching functions (for example, {overscore (x)}(λ),{overscore (y)}(λ),{overscore (z)}(λ)), then the values R, G and B are called tristimulus values and represented by X, Y and Z, respectively. If SRr,SRg,SRb represent the spectral response set of a film system, then the values R, G and B are commonly referred to as relative exposure values or, for simplicity, relative exposures. In either case, the values R, G and B offer a measure of the contents of red, green and blue light, respectively, which is reflected off the object and enters the capture system. These values can be converted into fractional quantities with the aid of the following expressions:
                                                        r              =                              R                                  R                  +                  G                  +                  B                                                                                                        g              =                              G                                  R                  +                  G                  +                  B                                                                                                        b              =                              B                                  R                  +                  G                  +                  B                                                                                                              Eq            .                                                  ⁢            2                    ⁢          a                ,                  2          ⁢          b                ,                  2          ⁢          c                    Those skilled in the art refer to r, g and b as chromaticity coordinates. From Equations 2a, 2b and 2c it is clear that:r+g+b=1  Eq. 3
A chromaticity diagram characterizes any set of colors by plotting one chromaticity coordinate versus another (g versus r, in this example), for each color in the set. If a chromaticity diagram displays the points that correspond to all monochromatic sources of light in the visual portion of the electromagnetic spectrum, the figure formed encompasses all physically realizable colors, and it is called the spectral locus. A comparison of chromaticity diagrams which characterize two distinct capture systems conveys important information regarding the color reproduction capabilities of each imaging system.
The color accuracy of a particular photographic system is defined according to how closely that system matches the HVS. This match can be measured with the computation of the average CIE (Commision Internationale De L'Eclairage) 1976 (L*a*b*) color difference ({overscore (ΔE)}*ab) for a diagnostic color patch set containing N patches. (A preferred set is the diagnostic color patch set disclosed in Appendix A of U.S. Pat. No. 5,582,961, which is incorporated herein by reference. The test colors of this set consist of 190 entries of known spectral reflectance specified at 10 nm increments, as set forth in the aforementioned Appendix.) {overscore (ΔE)}*ab is calculated according to Equation 4:
                                                        Δ              ⁢                                                          ⁢              E                        _                    ab          *                =                                            ∑                              l                =                1                            N                        ⁢                          Δ              ⁢                                                          ⁢                              E                                  ab                  ,                  i                                *                                              N                                    Eq        .                                  ⁢        4            
The color difference value for each patch,
      Δ    ⁢                  ⁢          E              ab        ,        i            *        ,is calculated between the 1976 CIE (L*a*b*)-space (CIELAB space) coordinates for each patch and the 1976 CIE (L*a*b*)-space coordinates which correspond to a transformation of the exposure signals captured by the photographic element.
                              Δ          ⁢                                          ⁢                      E                          ab              ,              i                        *                          =                                                            (                                                      L                                          TPS                      ,                      i                                        *                                    -                                      L                                          HVS                      ,                      i                                        *                                                  )                            2                        +                                          (                                                      a                                          TPS                      ,                      i                                        *                                    -                                      a                                          HVS                      ,                      i                                        *                                                  )                            2                        +                                          (                                                      b                                          TPS                      ,                      i                                        *                                    -                                      b                                          HVS                      ,                      i                                        *                                                  )                            2                                                          Eq        .                                  ⁢        5            The index TPS refers to “transformed photographic system”, while HVS refers to “human visual system”. The (L*a*b*) coordinates used in Equation 5 are computed as follows:
                                                                        L                                  HVS                  ,                  i                                *                            =                              116                ·                                                                                                    Y                        i                                                                    Y                        n                                                              -                    16                                    3                                                                                                                        a                                  HVS                  ,                  i                                *                            =                              500                ·                                  [                                                                                                              X                          i                                                                          X                          n                                                                    3                                        -                                                                                            Y                          i                                                                          Y                          n                                                                    3                                                        ]                                                                                                                        b                                  HVS                  ,                  i                                *                            =                              200                ·                                  [                                                                                                              Z                          i                                                                          Z                          n                                                                    3                                        -                                                                                            Y                          i                                                                          Y                          n                                                                    3                                                        ]                                                                                                              Eq            .                                                  ⁢            6                    ⁢          a                ,                  6          ⁢          b                ,                  6          ⁢          c                                                                                            L                                  TPS                  ,                  i                                *                            =                              116                ·                                                                                                    G                        i                                                                    Y                        n                                                              -                    16                                    3                                                                                                                        a                                  TPS                  ,                  i                                *                            =                              500                ·                                  [                                                                                                              R                          i                                                                          X                          n                                                                    3                                        -                                                                                            G                          i                                                                          Y                          n                                                                    3                                                        ]                                                                                                                        b                                  TPS                  ,                  i                                *                            =                              200                ·                                  [                                                                                                              B                          i                                                                          Z                          n                                                                    3                                        -                                                                                            G                          i                                                                          Y                          n                                                                    3                                                        ]                                                                                                              Eq            .                                                  ⁢            7                    ⁢          a                ,                  7          ⁢          b                ,                  7          ⁢          c                    The tristimulus values and exposure values Xi,Yi,Zi,Ri,Gi,Bi are calculated according to Equations 1a, 1b and 1c. Also, the values Xi,Yi,Zi may or may not account for a color correction matrix. The values Xn,Yn,Zn correspond to the tristimulus values of the light source used. Lower values of {overscore (ΔE)}*ab indicate higher color accuracy for the transformed photographic system.
The problem of lack of color reproduction accuracy is pointed out in commonly-assigned U.S. Pat. Nos. 5,582,961 (Photographic Elements Which Achieve Colorimetricaly Accurate Recording) and 5,609,978 (Method For Producing An Electronic Image From A Photographic Element), which both issued in the name of Giorgianni et al. In these patents, Giorgianni et al. benchmarks commercially available color negative films against the 1931 CIE color-matching functions based not only on the criteria described in equations 1 through 7, but also on a parameter that quantifies the level of noise introduced by transformation matrices. The comparisons performed demonstrate the lack of colorimetric accuracy between actual scenes and reproductions of those scenes on film systems. Giorgianni et al. describes a traditional photographic element that responds to light similarly to the human visual response to light. This is accomplished by having light sensitive records with sensitivity curves that are similar, in shape, to color-matching functions. After chemically processing the photographic element, the image content is transferred to a computer via a scanning device. In the computer, a matrix is applied to the image signals with the intent of further improving color reproduction accuracy, before the signal is finally sent to a display device. An electronic capture system with response curves that emulate color-matching functions would be highly advantageous over the system described in the aforementioned U.S. Pat. No. 5,609,978, since the electronic capture system would not require any chemical processing or scanning, and would significantly reduce the cost involved in practicing the disclosed method.
A second limitation that pertains to prior art photographic systems and capture devices in general relates to the gamut of colors that can be captured by those systems. FIGS. 2A and 2B evaluate the performance of two distinct capture systems: a typical motion picture color negative film and a typical high definition video camera (HD Video Rec. 709). FIG. 2A shows exposure error curves 22, 24 and 26 for red, green and blue exposure values computed with a typical set of color negative film response curves, relative to tristimulus values computed with the 1931 CIE color-matching functions. The red, green and blue errors are measured on the y-axis, in stops. Each point on the x-axis corresponds to one of the 190 patches described in Appendix A of the aforementioned U.S. Pat. No. 5,582,961. The solid horizontal lines 28 and 30 determine the +0.5 stop and −0.5 stop error levels, respectively, whereas the solid horizontal lines 32 and 34 determine the +⅙ stop and −⅙ stop error levels, respectively. FIG. 2B performs the same exposure comparison, now showing exposure error curves 36, 38 and 40 for red, green and blue exposure values computed for a typical high definition video camera relative to the human visual system, represented by the 1931 CIE color-matching functions. In both cases, given exposure values R, G, B and tristimulus values X, Y, Z that are computed according to equations 1a, 1b and 1c, the red, green and blue errors are computed according to equations 8a, 8b and 8c, respectively, with tungsten 3200 K° as the light source chosen.
                                                                        Error                R                            =                                                1                                                            log                      10                                        ⁡                                          (                      2                      )                                                                      ·                                                      log                    10                                    ⁡                                      (                                          R                      X                                        )                                                                                                                                          Error                G                            =                                                1                                                            log                      10                                        ⁡                                          (                      2                      )                                                                      ·                                                      log                    10                                    ⁡                                      (                                          G                      Y                                        )                                                                                                                                          Error                B                            =                                                1                                                            log                      10                                        ⁡                                          (                      2                      )                                                                      ·                                                      log                    10                                    ⁡                                      (                                          B                      Z                                        )                                                                                                                                Eq            .                                                  ⁢            8                    ⁢          a                ,                  8          ⁢          b                ,                  8          ⁢          c                    FIGS. 2A and 2B show that several patches, when captured by the film system or the video camera, contain large error values. This implies that a large number of colors that can be discriminated by the human visual system cannot be accurately captured by either system, with the gamut being less for video than film image capture systems. Therefore, an electronic capture device with a color gamut larger than what is currently available is highly desirable.
A third problem, particular to electronic or hybrid capture devices known in the prior art, relates to the level of noise introduced during processing of the image signals. The majority of the operations performed in the image processing path involves the application of matrices and/or look-up tables (LUTs) to input image signals. Look-up tables require intense computational power, since the volumes of image data in applications related to the motion picture industry are, in general, very large. Matrices are less computationally intensive. Either method can introduce considerable noise to the original signals. For matrices, the level of noise introduced depends on the magnitude of the coefficients used.
The color reproduction and noise problems described in the previous paragraphs point to the need for an electronic capture device with the following features:                A set of spectral sensitivity curves that captures exposures relative to a set of visual RGB color matching spectral responsivities.        A color gamut superior to what is offered by electronic video capture devices and traditional photographic systems based on currently available technology.        An image-processing path that limits the level of introduced signal noise to low thresholds.        
In commonly assigned U.S. Pat. No. 5,668,596 (which issued Sep. 16, 1997 in the name of R. M. Vogel and is entitled “Digital Imaging Device Optimized for Color Performance”), the patentee recognizes that it is desirable that the overall spectral sensitivities of the camera correspond to a set of all positive color matching functions that can be derived from the spectral response of the human eye via a linear 3×3 transformation. If these requirements are met, the camera will be able to discern color information in the scene in much the same way that a human observer would. Failure to achieve this goal will result in color reproduction errors. Accordingly, Vogel optimizes the capture device for color reproduction by a pre-defined combination of spectral response curves and a color-correction matrix. In this manner, greater color reproduction accuracy can be achieved in a digital camera by the combination of a set of spectral response curves modeled after all-positive color-matching functions and a color-correction matrix. The color correction matrix coefficients are computed in order to provide optimization of color reproduction for a unique combination of image sensor, passive optical elements and light source.
Notwithstanding the efforts in the prior art, there remains a need for an improved method and capture system which provide ways of solving the color reproduction problems described. In particular, attention must be devoted to solving these problems in better and more cost-effective ways than those provided by the prior art.